We have previously observed that the concentrations of the aromatic amino acids tryptophan, tyrosine and phenylalanine in human and rat serum of plasma exhibit characteristic daily fluctuations, which are largely the result of cycles in food consumption. The nocturnal rise in plasma tryptophan (or the rise induced by giving fasted rats access to carbohydrates) is associated with parallel increases in brain tryptophan, and at the rate at which the brain transforms this amino acid to the neurotransmitter serotonin. We now hope to determine 1) whether concentrations of other amino acids in brain, and of various amino acids in other tissues, also exhibit diet-dependent daily rhythms; 2) whether physiologic changes occur in brain tyrosine, and, if so, whether these cause parallel changes in brain catechol synthesis; 3) whether serotonin synthesis in other tissues (pineal and gut) also varies with tryptophan concentrations: 4) whether slices prepared from brains that contain more serotonin (e.g., from carbrohydrate-fed vs, protein-fed rats) also release more of the neurotransmitter in response to electric field stimulation; 5) why plasma tryptophan increases after insulin in rats, and fails to decline significantly in humans (i.e., its relation to plasma NEFA concentrations); and 6) the extent to which precursor (i.e., tryptophan) availability interacts with other factors in the physiologic control of serotonin synthesis. We have previously noted that protein-malnourished rats (e.g., weanlings of mothers given protein-poor diets; young rats given corn as their sole protein source) have apparently specific impairments in brain monoamine concentrations. We now wish to extend our characterization of these neurochemical deficits, and to examine associated changes in monoamines in other tissues.